1. Field of the Invention
The present invention relates to a two-group zoom photographic lens whose lens length and lens diameter are small, which includes a standard angle of view and which varies magnification from a wide angle to a long focal point.
2. Description of the Prior Art
Recently, an attempt has been rapidly made to form compact a so-called wide-angle lens having a variable power zone about a standard angle of view.
Among these, two-component wide-angle zoom lens, whose focal length is in the range of from 35 to 70 mm and F number in the range of from 3.5 to 4, has been used widely. However, even in such a zoom lens which has good performance and is relatively compact, the full length thereof is considerably long.
For example, in Embodiment 1 of Japanese Patent Application Laid-Open No. 43619/81, the minimum focal length is fw=36 mm and the maximum focal length is 68.5 mm whilst the length of the lens is wide angle L.sub.w =128=3.55 f.sub.w, and long focal point L.sub.T =113=3.14 f.sub.w. If the lens is intended to be formed into compactness without modification, the performance thereof unavoidably deteriorates.
In Japanese Patent Application Laid-Open No. 64811/84, an aspherical surface having a converging function is provided in a part of a negative lens in the front group to form the lens into compactness with a simple structure. In this lens, since the synthesized focal length of the negative lens in the front group is set to be long, the full length of the lens at the end of the long focal point is considerably small whereas the movement amount of zoom increases and the full length of the lens at the end of the short focal point is considerably great. Accordingly, the shortest focusing length of focusing by the lens in the front group cannot be made small. In the configuration of the lens in the rear group of the aforesaid lens, it is not possible to increase the refracting power and thus it is not possible to reduce the focal length of the negative lens in the front group.
In Japanese Patent Application Laid-Open Nos. 111013/83 and 132207/83, a teleconverter is added to the rear portion of a two-component zoom lens to make both length and diameter compact. However, this makes the lens into a three-component type, which not only complicates a mechanism but unavoidably complicates errors or the like in variation of aberrations resulting from variation in spacing between the lenses due to zooming as compared with the two-component type.
The lens disclosed in Japanese Patent Application Laid-Open No. 114236/79 has a simple configuration in which negative lenses in the front group comprise two groups and two or three lenses, an positive lenses in the rear group comprise four to five lenses but involves a problem in that the full length of the lens is great. For example, in Embodiment 1 of said patent, the short focal length is f.sub.w =43.8 and the long focal length is f.sub.T =72.8 while the focal length of the front group lens is long, f.sub.f =-91. Because of this, the lens length at the long focal point is relatively small, L.sub.T =98.6 while lens extension at zooming or focusing is great and the lens length at the short focal point is great, L.sub.w =122.0. On the other hand, the focal length of the front group lens is small, f.sub.f =-62.5 and the extension is small. In Embodiment 3, the lens length is f.sub.w =41.0 and the focal length is f.sub.T =73.5 while the full length of the lens is L.sub.w =121.5 and L.sub.T =115.0, which cannot be said small. This results from the fact that the number of lenses in the rear group increases due to correction for aberrations to increase the size of lens accordingly.
Furthermore, Japanese Patent Application Laid-Open No. 121011/83 is being disclosed as an example which uses an aspherical surface. According to this lens, an aspherical surface is introduced into a negative lens in the front group to provide compactness and high performance. However, this lens is large in consideration of L.sub.w =140.8 and L.sub.T =137.2 while f.sub.w =28.7 and f.sub.T =81.6 and of approximately 3 of zoom ratio. Moreover, correction for aberrations is not sufficient due to the remaining aberrations caused by the lens in the rear group.